This invention relates to a scanning electron microscope (SEM), in particular a scanning electron microscope operating under a slightly elevated pressure, or retrofitting a scanning electron microscope that operates in vacuo for operation with gas in the specimen chamber, and in particular it relates to an improved detection efficiency of such a microscope (i.e., improving the signal-to-noise ratio of the images recorded with it), in particular in operation with a low primary energy.
With a scanning electron microscope that operates under a slightly elevated pressure (pressure SEM), a maximum operating pressure of a few hectopascals to a few kilopascals is usually allowed in the specimen chamber. At this pressure, the primary electrons have only a short mean free path length. Therefore, the microscope column is sealed with respect to the specimen chamber by a pressure stage aperture (or a pressure limiting aperture) through which the primary electron beam enters the specimen chamber. Above this pressure stage aperture, the pressure is reduced by several powers of ten.
The backscatter electrons emitted by the specimen can be detected with a scintillator-light guide combination arranged between the specimen and the pressure stage aperture. An improved resolution, however, is obtained when using the secondary electrons which are emitted by the specimen and can be detected with the help of a collector electrode (PCT Patent application WO 88/09564 A1). The bottom side of the pressure stage aperture is usually designed as a collector electrode or a separate collector electrode may be arranged beneath the pressure stage aperture.
With other scanning electron microscopes which operate under a slightly elevated pressure, the secondary electrons are detected through the opening in the pressure stage aperture in a type of antechamber which is sealed toward the top with respect to the lens by another pressure stage aperture. Here again, a collector electrode is used as the detector for the secondary electrons (PCT Patent application WO 90/04261 A1). Although this design has been tested (G. D. Danilatos, xe2x80x9cDesign and Construction of an Environmental SEM; Part 4xe2x80x9d Scanning, vol. 12 (1990) p. 23), it has not been successful in practice.
Detector systems with collector electrodes have a poor detection sensitivity because of the noise with the subsequent electronic amplification, and therefore they require pre-amplification of the secondary electron signal before it reaches the collector electrode. This pre-amplification takes place with the help of an electric field between the specimen and the collector electrode which accelerates the secondary electrons emitted by the specimen so that they can ionize gas molecules. After colliding with the gas molecules, the secondary electrons thus generated in the gas and the secondary electrons already present previously are accelerated again through the electric field and generate additional secondary electrons in the gas. In this way, a secondary electron cascade is induced by the secondary electrons emitted by the specimen and ultimately reaches the collector electrode. Even when using a light guide with a downstream photomultiplier as the gas scintillation detector, a secondary electron cascade is used as the pre-amplifier.
Despite this cascade pre-amplification, in both cases the signal-to-noise ratio of the images recorded at a slightly elevated pressure are much worse at the same beam amperage than with the images recorded with conventional secondary electron detectors without elevated pressure. Therefore, improving detection efficiency and reducing detector noise of pressure SEMs are important goals when investigating sensitive specimens in particular (e.g., semiconductor components, plastics, biological and medical specimens).
In addition, the use of a low primary energy is also advantageous in investigating sensitive specimens so that less energy is applied to the specimen and damage to the specimen due to the electron beam is limited to a thin surface layer. The pressure SEMs known so far need a secondary electron cascade in the gas for their collector electrode and are therefore unsuitable for operation with a low primary energy (1 keV, for example), in particular in observation of wet specimens. For operation with a low primary energy, the shortest possible gas path between the specimen and the pressure stage aperture above it is necessary, as well as the lowest possible pressure above the pressure stage aperture, because with a lower primary energy, the mean free path length of the primary electrons in the gas also decreases. Under these conditions, however, no satisfactory cascade pre-amplification is possible so that the pressure SEMs known in the past can be used in operation with gas in the specimen chamber for observation of wet specimens only above a primary energy of 3 keV. However, even at 3 keV a signal background is produced due to the large amount of scattered primary electrons, resulting in an even worse signal-to-noise ratio in the images than at a higher primary energy. The same is also true in operation with a low primary energy (1.5 keV, for example) which is used with the pressure SEMs known in the past at a pressure up to approximately 1.5 hPa for the purpose of combatting a charge buildup.
The pressure SEMs known today are also not very suitable for a low beam amperage due to the poor detection efficiency and the time constant of the operational amplifier even when using a collector electrode. When investigating wet specimens at a high magnification in particular, a lower beam amperage and a lower primary energy would be important, however, to prevent local heating and the resulting drying out of the specimen location observed.
In addition, there is a demand for a pressure SEM with a good signal-to-noise ratio which would also be suitable for a low primary energy to permit better reproduction of fine surface structures and to prevent the edge effect.
Preventing a charge buildup in operation with a low primary energy is another important problem which is not solved satisfactorily by today""s pressure SEMs because of the resulting poor signal-to-noise ratio. Possible applications for corresponding pressure SEMs include, for example, imaging sensitive plastics, use in electron beam lithography and in metrology equipment, such as that used in the semiconductor industry for automated monitoring in production. Instead of that, the influence of charge buildup is reduced today in other metrology equipment by using backscatter electrons for imaging.
The object of this invention is to provide an improved SEM that operates under a slightly elevated pressure (hereinafter: pressure scanning electron microscope or pressure SEM) which does not have the above disadvantages of traditional pressure SEMs, and in particular to improve the detection efficiency of pressure SEMs, where detection takes place through the pressure stage aperture through which the microscope column is closed with respect to the specimen chamber (or the signal-to-noise ratio of the images recorded with it), in particular in operation with a low primary energy.
This object is achieved by an SEM having the features according to Patent Claim 1. Advantageous embodiments of this invention are defined in the dependent claims.
This invention is based in general on the idea of improving upon a scanning electron microscope with the features according to the definition of the preamble of Patent Claim 1 so that a high-sensitivity detector under a positive bias with respect to the specimen is used as the detector.
According to this invention, this object is achieved according to a first aspect in particular by the fact that one or more electrode elements (solid electrodes or thin electrode layers) are arranged above the pressure stage aperture and are at a positive potential with respect to the pressure stage aperture, not using a collector electrode as the detector for the secondary electrons generated in the specimen and in the gas but instead using one or more detectors with a higher detection sensitivity.
The advantage of this invention consists in particular of the fact that, independently of the presence of a secondary electron cascade, a high detection efficiency is achieved for the secondary electrons emitted by the specimen and a low noise of the detector system. With the pressure SEMs according to this invention, the length of the path the primary electrons must travel through the gas can be made very short ( less than 300 xcexcm) by using a short distance between the specimen and the pressure stage aperture and by pumping out sharply above the pressure stage aperture. However, the pressure SEMs known so far are not suitable in principle for such short path lengths ( less than 300 xcexcm) of the primary electrons through the gas, because an adequate secondary electron cascade cannot develop either above or below the pressure stage aperture.
With the good suitability for a short path length of the primary electrons through the gas, in particular two important new fields of applications for pressure SEMs are developed through the pressure SEMs according to this invention and the properties of known applications are improved.
First, as a new application, scanning electron microscopy can be performed at a low primary energy (e.g., 1 keV or lower) with gas in the specimen chamber (e.g., water vapor at a pressure of 10 hPa) with no problem. Because of the high detection efficiency and the low noise of the detector system, only a low beam amperage is necessary. Thus, wet specimens can be observed at higher magnifications than before without the drying out the observed specimen location. In addition, an important improvement on the previous options is achieved by combatting charge buildup in operation with a low primary energy and a good detection efficiency.
Secondly, scanning electron microscopy can be performed with secondary electrons at an unusually high pressure ( greater than 100 hPa) with the pressure SEMs according to this invention at a high primary energy ( greater than 15 keV). When using a pressure stage aperture with a very small bore diameter (of 20 xcexcm, for example), it is even possible to operate these scanning electron microscopes at ambient pressure (1013 hPa) with extremely small distances ( less than 50 xcexcm) between the specimen and the pressure stage aperture.
In addition, especially at a low primary energy, contamination of the specimen, which is a problem in scanning electron microscopy at a low primary energy, can be counteracted with the help of certain gases (e.g., oxygen or compounds containing oxygen). (Effective purification of the specimen can before examination by scanning electron microscopy be achieved if a high-frequency plasma purification device is integrated into the air lock.)
According to a second aspect of the present invention, a further improvement in the detection efficiency of pressure SEMs, where detection of the secondary electrons takes place through one or more pressure stage apertures, is achieved in particular by increasing the amount of secondary electrons passing through the pressure stage apertures. This component can be increased by constructing at least one pressure stage aperture in layers, namely two or more conductive layers with insulating layers or poor conductors between them, so that the top and bottom sides of the pressure stage aperture can be set at different potentials so that an electric field is created in the pressure stage aperture to improve the transport of the secondary electrons.
The first and second aspects of this invention described above may be implemented jointly or individually to solve the object defined above. In both cases, acceleration of secondary electrons from an area near the specimen with a high pressure to the detector in an area remote from the specimen with a reduced pressure (vacuum) is supported.
With the usual secondary electron detectors with a high detection sensitivity, each individual secondary electron triggers a large number of photons, electrons or electron-hole pairs which are then detected. To do so, it is necessary to supply energy to the secondary electrons before they reach the detector or (with the channel plate or channeltron) along the detector surface. To this end, a high electric voltage must be applied between the specimen and the detector or (in the case of channel plate and channeltron) along the detector surface.
A preferred embodiment of the present invention therefore consists of the fact that the entire detector surface or parts of the detector surface are at a potential which is positive in comparison with the potential of the specimen by more than 500 V, preferably 1000 V. Use of such detectors in the specimen chamber of a traditional pressure SEM would lead to sparkover.
A preferred embodiment of the present invention consists of the fact that the pressure SEM has a combined electrostatic and magnetic lens. In comparison with the purely electrostatic lens (German Patent application DE 3703029 A1) which is easier to manufacture, this is an advantage in particular when the pressure SEM designed in this way is also to be used at both conventional primary energies above 3 keV. In comparison with a purely magnetic lens, a combined electrostatic and magnetic lens is an advantage in particular when the secondary electrons are to be focused through an electric field at the orifice of a pressure stage aperture. Embodiments with a single-pole magnetic lens (e.g., according to the lenses described in European Patent application EP 0790634 A1, German Patent application DE 4236273 A1, European Patent applications EP 0767482 A2 and EP 0817235 A1) yield especially good resolution and an especially good detection efficiency as important advantages, in particular for investigating non-magnetic specimens.
The improved detection efficiency comes about due to an increase in the amount of secondary electrons generated in the specimen and in the gas and reaching the detector through the pressure stage apertures. When using a single-pole magnetic lens, this component is increased because the secondary electrons travel on spiral paths around the magnetic field lines passing through the orifices of the pressure stage apertures. In the gas of the specimen chamber, the secondary electrons are moved by collisions to new spiral paths around adjacent field lines, although these usually also pass through the orifices of the pressure stage apertures when the magnetic field strength is sufficient.
Implementation of this invention is not limited to pressure SEMs but instead it can also be implemented by retrofitting SEMs that would normally be operated with the specimen chamber evacuated. The ease in converting the equipment between the two applications is one advantage of this invention.